The Substrate
The Athabasca tar sands in Alberta are estimated to contain at least 1.7 trillion barrels of oil, and as such may represent around one-third of the world's total petroleum resources. Over 85% of known bitumen reserves lie in this deposit, and their high concentration makes them economically recoverable. Other significant deposits of tar sands exist in Venezuela and the USA, and similar deposits of oil shale are found in various locations around the world. These deposits consist of a mixture of clay or shale, sand, water and bitumen. Bitumen is a viscous, tar-like material composed primarily of polycyclic aromatic hydrocarbons (PAHs). Extraction of the useful bitumen in tar sands is a non-trivial operation, and many processes have been developed or proposed. Lower viscosity deposits can be pumped out of the sand, but more viscous material is generally extracted with superheated steam, using processes known as cyclic steam stimulation (CSS) or steam assisted gravity drainage (SAGD). More recently, this latter technology has been adapted to use hydrocarbon solvents instead of steam, in a vapor extraction (VAPEX) process. Supercritical fluids (SCFs) have been considered a potentially attractive extractant for bituminous deposits since the 1970s. Their low densities and low viscosities make them particularly effective at permeating tar sands and oil shales and extracting organic deposits, and the energy costs associated with the moderate temperatures and pressures required to produce them compare very favourably with those processes that use superheated steam. For example, bitumen has been successfully recovered from Stuart oil shale in Queensland using supercritical carbon dioxide (scCO2), and from Utah oil sands using supercritical propane (sc propane). Very recently, Raytheon announced the use of scCO2 in combination with RF heating to extract oil shale deposits beneath Federal land in Colorado, Utah and Wyoming.
Bitumen typically contains around 83% carbon, 10% hydrogen and 5% sulfur by weight, along with significant ppm amounts of transition metals like vanadium and nickel associated with porphyrin residues. This low-grade material commonly needs to be converted into synthetic crude oil or refined directly into petroleum products before it can be used for most applications. Typically, this is carried out by catalytic cracking, which redistributes the hydrogen in the material. Catalytic cracking produces a range of ‘upgraded’ organic products with relatively high hydrogen content, but leaves behind a substance known as asphaltene, which is even more intractable than bitumen and contains very little hydrogen. Unless this asphaltene is upgraded by reaction with hydrogen, it is effectively a waste product.